A variety of sensors are used in modern drilling operations, particularly in the search for oil and natural gas. Such sensors can include accelerometers and magnetometers (for directional or orientation sensing, such as to identify magnetic North and/or a gravity vector) and radiation sensors (to determine rock properties). These sensors are combined with electronic circuits that can amplify and process the electrical signals and together form a sensor package. The processed data generated by a sensor package are transmitted in real-time to the surface (e.g., using a mud-pulse telemetry system). The process of drilling while using such a sensor package is called logging while drilling (LWD) and can be used to determine the direction in which the drill-bit is headed and the type of rock encountered. The data from the tool can be analyzed by geologists while drilling and employed to make informed decisions about how deep to drill a vertical well or for steering the drill-bit in directional-drilling operations. This process is known as geosteering.
Radiation sensors are generally of two types: scintillation and proportional. Scintillation crystals are used to detect gamma radiation by converting incident gamma photons into a brief flash of light. The light pulse may be detected by a photomultiplier that is optically coupled to the crystal, and whose output is an electrical pulse. A typical example of scintillator material is a single crystal of Sodium Iodide doped with Thallium. An alternative radiation sensor is the proportional (gas-filled tube) type that directly outputs an electrical pulse. Yet another possible alternative is a bulk semiconductor type such as a Lithium-drifted Germanium sensor. A particular application of such a radiation sensor is in logging tools to determine the natural radioactive properties of rock formations encountered by a borehole.
Sensor instruments like those used to detect orientation and/or radiation are subjected to extreme mechanical shock and vibration during the drilling process. However, typical accelerometers, magnetometers, scintillator crystals, photomultipliers, and the like are inherently rather fragile. The result is that these expensive components can fail during drilling operations, resulting in very expensive down-time of the drilling rig to retrieve the logging tool from the well. Furthermore, drilling technology advances, such as fast drilling and under-balanced drilling, have increased the shock levels imposed on the sensors.
While the invention is susceptible to various modifications and alternative forms, specific embodiments of the invention are provided as examples in the drawings and detailed description. It should be understood that the drawings and detailed description are not intended to limit the invention to the particular form disclosed. Instead, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.